1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
A conventional image forming apparatus such as a copying machine incorporates a heat-roller type fixing unit. The fixing unit includes a heat roller and a pressure roller. The heat roller incorporates a heat source such as a heater. The pressure roller is in pressure contact with the heat roller and is driven by the heat roller. A print medium passes through a nip formed between the heat roller and the pressure roller so that a toner image on the print medium is fused by heat under pressure. The heat source is controlled to generate a controlled amount of heat such that the surface temperature of the heat roller is optimum for fusing a toner image on a variety of types of print media.
FIG. 15 illustrates a configuration of an image forming section of a conventional image forming apparatus.
FIG. 16 illustrates a control block diagram of the conventional image forming apparatus.
Referring to FIG. 15, an image forming apparatus such as an electrophotographic printer and a copying machine incorporates an image forming section 20 and a fixing unit 24. The image forming section 20 includes a photoconductive drum 21, a charging unit 31, an LED (light emitting diode) head 33, a developing unit 32, and a transfer unit 22. The charging unit 31 charges the surface of the photoconductive drum 21 uniformly and the LED head 33 irradiates the charged surface of the photoconductive drum 21 with light in accordance with print data to form an electrostatic latent image on the photoconductive drum 21. The developing unit 32 applies toner to the electrostatic latent image to develop the electrostatic latent image into a toner image. The transfer unit 22 takes the form of, for example, a transfer roller and opposes the photoconductive drum 21 and transfers the toner image onto a print medium 23 that is being transported by a transporting unit, not shown. The fixing unit 24 includes fixing rollers that are in pressure contact with each other so that one of the fixing rollers drives the other in rotation.
A print controller 11 in FIG. 16 controls the operations of the image forming section 20 and fixing unit 24. The print controller 11 takes the form of a computer that includes primarily a microprocessor, an I/O port, and a timer, and a memory means such as a ROM and a RAM. The print controller 11 receives a print job from an external host apparatus such as a personal computer. The host apparatus generates a video signal that includes print data in bit map form and a control signal that commands printing of the print data, and sends the video signal and the control signals to the print controller 11. In accordance with the video signal and control signals, the print controller 11 controls the operations of the image forming section 20 and the fixing unit 24.
The print controller 11 is connected primarily to the charging unit 31, LED head 33, developing unit 32, the transfer unit 22, fixing unit 24, a remaining-medium sensor 30, and a temperature detector 15. The remaining-medium sensor 30 detects a remaining number of pages of the print medium 23. The temperature detector 15 detects the temperatures of, for example, the heat roller. Upon receiving a print command from the host apparatus, the print controller 11 checks the detection signal output from the temperature detector 15 to determine whether the temperature of the fixing unit 24 is within a predetermined range. If the temperature is out of the predetermined range, then the print controller 11 causes current to run through the heater 24a until the temperature is within the predetermined range.
Then, the print controller 11 supplies a voltage to the charging unit 31 which in turn charges the surface of the photoconductive drum 21. The print controller 11 then checks the detection signal outputted from the remaining-medium sensor 30 to determine whether the print medium 23 exists in the image forming apparatus. If the print medium 23 is available, the print controller 11 initiates to feed the print medium 23 to the image forming section 20.
When the print medium 23 reaches a predetermined position in a transport path, the print controller 11 sends timing signals including an advance direction sync signal and a traverse direction sync signal to the host apparatus. As previously described, the host apparatus generates a video signal that includes print data in bit map form, and sends the video signal to the print controller 11 on a line-by-line basis in synchronism with the timing signal.
The print controller 11 sends the received video signal to the LED head 33, the lines of the video signal being sent sequentially in synchronism with a clock signal. Once the print controller 11 has transmitted print data for one line to the LED head 33, the print controller 11 drives the LED head 33 to hold the print data for one line.
Then, print controller 11 drives the LED head 33 to energize LEDs in the LED head 33 in accordance with the print data for one line, so that the LEDs emit light to form an electrostatic latent image.
The LED head irradiates the surface of the photoconductive drum 21, negatively charged by the charging unit 31, with light. Charges in irradiated areas on the photoconductive drum 21 are dissipated so that the irradiated areas increase in potential to substantially zero volts to form dots on the photoconductive drum 21. The dots in turn form an electrostatic latent image as a whole. The developing unit 32 supplies negatively charged toner to the dots on the photoconductive drum 21 to form a toner image. As the photoconductive drum 21 rotates, the toner image is transported to the transfer unit 22 where the toner image is transferred onto the print medium 23 that passes a transfer point defined between the photoconductive drum 21 and the transfer unit 22.
The print medium 23 then passes through the fixing unit 24, so that the toner image on the print medium 23 is fused into a permanent image. The print medium 23 further advances to be discharged to a stacker outside of the image forming apparatus.
FIG. 17 illustrates a transport path of the print medium 23 in the conventional image forming apparatus.
As shown in FIG. 17, the image forming apparatus incorporates a paper cassette 34 in which a stack of print medium 23 is held therein. The paper cassette 34 has a paper guide 35 that aligns the print medium 23. When the print controller 11 initiates to feed the print medium 23 from the paper cassette 34, a top page of the stack of the print medium 23 is fed from the paper cassette 34 to the image forming section 20. When the medium-feed detector 25, disposed somewhere in the transport path of the print medium 23, detects the print medium 23, the print controller 11 drives the LED head 33 to initiate formation of an electrostatic latent image on the photoconductive drum 21. Subsequently, the print medium 23 is transported through the transfer unit 22 and then the fixing unit 24, and discharged to the outside of the image forming apparatus. A discharge sensors 26 and 27 are disposed downstream of the fixing unit 24 with respect to the direction of travel of the print medium 23 and detect the print medium 23.
FIGS. 18A–18B illustrate examples of configuration of the fixing unit 24 according to the conventional image forming apparatus.
FIG. 18A illustrates a configuration of the roller type fixing unit 24.
Fixing rollers each include an aluminum pipe 37 and a rubber layer 36 formed on the surface of the aluminum pipe 37. The fixing rollers are aligned vertically and rotated by a motor, not shown. The fixing rollers are heated by the heaters 24a so that the toner image on the print medium 23 is fused by heat under pressure.
FIG. 18B illustrates a configuration of the belt type fixing unit 24. The belt type fixing unit 24 uses a fixing belt 38. The nip 40 has a larger area when it is formed between the lower fixing roller and the fixing belt 38 than when it is formed between the upper and lower fixing rollers. Therefore, a larger amount of heat per unit time can be supplied to the print medium 23. The temperature detector 15 is disposed at a position X1 on the surface of the upper left roller. The nip 40 is at a position X2.
FIG. 19 is a graph of temperature versus time length during which the heater of the conventional roller type fixing heater is energized.
In order that the roller type fixing unit 24 has a large nip, the rubber layer 36 should be thick. For this reason, the roller type fixing unit 24 includes rollers usually having a large heat capacity. A large heat capacity requires a longer time for heat to be transferred. This delay time in heat transfer is a time length from when the heater 24a begins to be energized to supply heat energy to the fixing roller until the surface temperature of the fixing roller starts to increase. The delay time is usually several seconds for the conventional roller-type fixing unit 24.
A belt-type fixing unit 24 also requires the rubber layer 36 and therefore the belt type fixing unit 24 also includes rollers usually having a large heat capacity. For the belt type fixing unit 24, the distance X2−X1 between the temperature detector 15 and the nip 40 is long and thus it will be some time before the temperature at the nip 40 reaches the temperature detected by the temperature detector 15.
FIGS. 20A–20D illustrate the positions of print medium 23 within the image forming apparatus and show a delay time in detecting the temperature of the conventional belt type fixing unit 24.
FIG. 20A illustrates the position of the print medium 23 before a printing operation is initiated. FIG. 20B illustrates the position of print medium 23 when the leading end of the print medium 23 is a distance X2−X1 (FIG. 18B) upstream of the nip 40 in the fixing unit 24.
The temperature detector 15 detects the temperature in an area A on the belt 38 in FIG. 20B. The area A on the belt 38 will come into contact with the print medium 23 when the print medium 23 reaches the fixing point in FIG. 20C. Then, the heat in the area on the belt 38 is lost to the print medium 23 and the area A will reach a position in FIG. 20D where the temperature detector 15 detects the temperature of the area again. This fact implies that there is a delay in temperature detection of an area A on the belt 38 from when the area A comes into contact with the print medium 23 until the area A rotates back to the temperature detector 15. The fixing belt 38 of the belt type fixing unit 24 has a longer circumferential length than the fixing roller of a roller type fixing unit. Therefore, a delay in temperature detection is longer in the belt type fixing unit 24 than in the roller type fixing unit 24. The delay in temperature detection causes a delay in detecting that the print medium 23 has been discharged from the fixing unit 24, supplying an excess amount of heat to the fixing unit 24.
The temperature control of the fixing unit 24 will be described.
FIG. 21 illustrates a temperature control for the fixing unit 24 of the conventional image forming apparatus.
FIG. 21 shows an acceptable temperature range in which the fixing unit 24 can perform a fixing operation normally. If a printing operation is initiated when the temperature of the fixing unit 24 is lower than the lower limit of the acceptable temperature range, the toner image on the print medium 23 is not fused sufficiently into the print medium 23. This causes “cold offset” where the toner does not adhere to the print medium 23 but to the fixing roller. Conversely, if a printing operation is initiated when the temperature of the fixing unit 24 is higher than the upper limit of the acceptable temperature range, the toner image on the print medium 23 is heated excessively to lose its viscosity. This causes “hot offset” where the toner does not adhere to the print medium 23 but to the fixing roller.
The acceptable temperature range is determined by conditions such as the environmental temperature, the type and thickness of the print medium 23, and the speed of travel of the print medium 23. The print controller 11 has a memory area that holds a table of the temperature settings corresponding to the conditions.
Based on the detection signal generated by the temperature detector 15, the print controller 11 turns on the heater 24a when the surface temperature of the fixing roller is below a target temperature, and turn off the heater 24a when the surface temperature is higher than the target temperature. The temperature control is performed in this manner so that the surface temperature of the fixing roller is as close to the target temperature as possible during printing.
The fixing operation during printing will be described. When printing is initiated, the print controller 11 checks the detection signal received from the temperature detector 15 to determine whether the temperature of the fixing unit 24 is within the acceptable temperature range. Then, the print controller 11 performs one of the following operations depending on the temperature of the fixing unit 24:
(1) If the current temperature of the fixing unit 24 is within the acceptable temperature range, the print controller 11 sets the target temperature to a value that corresponds to the condition of the print medium 23 to be printed on subsequently.
(2) If the current temperature of the fixing unit 24 is lower than the acceptable temperature range for the subsequent page, the print controller 11 sets the target temperature to a value that corresponds to the condition of the subsequent pages of the print medium 23. Then, the print controller 11 energizes the heater 24a so that the temperature of the fixing unit 24 is within the acceptable temperature range. As soon as the temperature of the fixing unit 24 is within the acceptable temperature range, the respective motors are rotated to initiate a printing operation.
(3) If the current temperature of the fixing unit 24 is higher than the acceptable temperature range for the subsequent page, the print controller 11 sets the target temperature to a value that corresponds to the condition of the subsequent page of the print medium 23. Then, the print controller 11 causes the fixing unit 24 to dissipate heat without turning on the heater 24a. As soon as the temperature of the fixing unit 24 is within the acceptable temperature range, the respective motors are rotated to initiate a printing operation.
However, with the aforementioned conventional image forming apparatus, the temperature of the fixing unit 24 may overshoot to be out of the acceptable temperature range. When the temperature of the fixing unit 24 is out of the acceptable temperature range, if the print controller 11 receives a print command and performs a printing operation, hot offset will occur to impair print quality.
FIG. 22 illustrates an example of overshoot of the temperature of the fixing unit 24 of the conventional image forming apparatus.
Referring to FIG. 22, when continuous printing of a plurality of pages of print medium 23 is performed, the heater 24a is energized from the initiation of printing until the completion of printing. Once printing has completed, the fixing unit 24 does not lose heat to the print medium 23, and causing the temperature of the fixing unit 24 to overshoot so that the temperature of the fixing unit 24 becomes out of the acceptable temperature range. If a print command for the subsequent print job is received at time ta, hot offset occurs due to an excessively high temperature of the fixing unit 24.
Of course, the fixing unit 24 will have lost an excessive amount of heat by time tb and printing can be performed normally if a print command for the subsequent print job is received at or after time tb. This, however, implies that a certain waiting time should be allowed before the next printing job is initiated, causing a decrease in the throughput of the image forming apparatus.